Polynuclear Single-Molecule Magnets (SMMs) comprise multiple exchange-coupled metal ions that can be collectively magnetized below a characteristic blocking temperature, TB. Up to now, low values of TB have hampered potential applications in data storage and quantum computing. When the exchange-coupled giant spin ground state is well separated from excited spin states, the SMM properties are dominated by magneto-crystalline anisotropy. However, in most cases, the constituent ions are coupled by relatively weak super-exchange interactions, leading to a separation between spin states of just a few kelvin. Recently, SMM properties were observed for a Fe6 molecule containing six iron ions strongly coupled via direct metal-metal bonds.[1] Here we present a spectroscopic investigation of this Fe6 molecule, where Frequency-Domain Fourier-Transform THz-EPR was combined with multi-frequency high-field EPR so that the magnetic anisotropy could be analyzed thoroughly. The experimental data suggest the ground state to be well separated, demonstrating the possibility of metal-metal bonded SMMs with drastically improved TB values.

[1] Sánchez, Betley, JACS 137,13949, 2015

*Work at the NHMFL is supported by the NSF (DMR-1157490 and DMR-1610226). J.N. thanks the DFG for funding (NE 2064/1-1 FOR).